Fission products system design

Off-Gas Treatment. Before the advent of the shear, the gases released from the spent fuel were mixed with the entire dissolver off-gas flow. Newer shear designs contain the fission gases and provide the opportunity for more efficient treatment. The gaseous fission products krypton and xenon are chemically inert and are released into the off-gas system as soon as the fuel cladding is breached. Efficient recovery of these isotopes requires capture at the point of release, before dilution with large quantities of air. Two processes have been developed, a cryogenic distillation and a Freon absorption. 

Some of the pyrochemical processes have more potential for being proliferation resistant because of the great similarity of the chemistry of uranium, plutonium, and some of the fission products in the chosen systems. Ordinary processes are designed to maximize differences in chemical behavior in order to separate constitutents. For some of the pyrochemical processes the chemical equilibria are such that partial separations are possible but complete separations are thermodynamically limited. For example, excess uranium can be separated from plutonium by precipitation in a molten metal such as zinc only until both are present in about equal quantities in solution, but no further ( 3, 4). Likewise, the solubility of fission products is selectively limited. Only a portion of elements such as ruthenium will stay in solution and be removed 05). The majority of the ruthenium precipitates with the actinides. A complete separation is again thermodynamically limited. As a result only a modest dependence needs to be placed on process equipment and facility design for proliferation resistance. 

However, the fact that the fuel is both mobile and unclad raises serious concerns about fuel leakage and containment and control of the gaseous fission products. These concerns will impact the designs of containment and fuel chemistry management systems, and may require significant developmental efforts to demonstrate. 

Review of Weqairements. It has been shown in Chap. 4 that the k of the MTR is very much larger than that of the early uranium-graphite reactors and that considerable excess reactivity will, b used in order to handle losses due to fuel depletion, experiments, fission product poisons, etc. Any control system designed for>such a reactnr not only must be able to maintain the machine at any given power but, when necessary, smst also be able to overcome quickly all the excess reactivity. 

A. General Design Criterion 41 (Reference...) as it relates to containment atmosphere cleanup systems being designed to control fission product releases to the reactor containment following postulated accidents. 

Barrier (against radioactive releases) Structure, set of structures or of systems which contrast the uncontrolled release of radioactive material to the outside or to the inside of a nuclear plant. For the radioactivity connected to fission products, the plant design provides the following barriers the fuel matrix, the fuel element claddings, the primary circuit(s), the containment system. 

To limit considerably the release of fission products beyond the containment, a permanent under-pressure is maintained in the inter-containment gap of the V-392 design. This safety function, one of the most important, is fulfilled by two systems (1) an exhaust ventilation system equipped with a filtering plant with suction from the inter-containment gap and outlet... 

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